1. Field of the Invention
The present invention relates to a luminescence display apparatus comprising electroluminescence elements and thin-film transistors.
2. Description of the Related Art
In recent years, electroluminescence (referred to hereinafter as EL) display apparatuses employing EL elements as emissive elements have attracted attention as being the display apparatuses to replace CRTs and LCDs, and the research and development also have advanced on EL display apparatuses comprising thin-film transistors (referred to hereinafter as TFT) as switching elements to drive the EL elements.
FIG. 1 shows an equivalent circuit of an EL display apparatus comprising a conventional EL element and TFT.
FIG. 1 is an equivalent circuit of an EL display apparatus comprising a first TFT 130, a second TFT 140, and an organic EL element 160, and shows the circuitry near a gate signal line Gn of row n and a drain signal line Dm of column m.
The gate signal line Gn supplying a gate signal and the drain signal line Dm supplying a drain signal are perpendicular to each other, and near the intersection of both signal lines are provided the organic EL element 160 and the TFTs 130, 140 driving the organic EL element 160.
The first TFT 130, which is a switching TFT, comprises gate electrodes 131 connected to the gate signal line Gn and supplied with gate signals, a drain electrode 132 connected to a data signal line (drain signal line) Dm and supplied with data signals, and a source electrode 133 connected to a gate electrode 141 of the second TFT 140.
The second TFT 140, which is an organic EL element driver TFT, comprises the gate electrode 141 connected to the source electrode 133 of the first TFT 130, a source electrode 142 connected to an anode 161 of the organic EL element 160, and a drain electrode 143 connected to a driving power supply 150 that is supplied to the organic EL element 160.
Furthermore, the organic EL element 160 comprises the anode 161 connected to the source electrode 142, a cathode 162 connected to a common electrode 164, and an emissive element layer 163 sandwiched between the anode 161 and the cathode 162.
Furthermore, a storage capacitor 170 is provided with one electrode 171 connected between the source electrode 133 of the first TFT 130 and the gate electrode 141 of the second TFT 140 and another electrode 172 connected to a common electrode 173.
The driving method of the circuit shown in the equivalent circuit of FIG. 1 will now be described. When the gate signal from the gate signal line Gn is applied to the gate electrode 131, the first TFT 130 turns on. As a result, the data signal from the data signal line Dm is supplied to the gate electrode 141 and the voltage of the gate electrode 141 becomes identical to the voltage of the data signal line Dm. A current proportional to the voltage value supplied to the gate electrode 141 is then supplied from the driving power supply 150 to the organic EL element 160. As a result, the organic EL element 160 emits light at an intensity in accordance to the magnitude of the data signal.
A conventional EL display apparatus will be described next with reference to FIGS. 2, 3A, and 3B. FIG. 2 is a top view showing one pixel of the conventional EL display apparatus. In FIG. 2, a gate signal line 51 corresponds to the gate signal line Gn, a data signal line 52 corresponds to the data signal line Dm, a driving power supply 53 corresponds to the driving power supply 150, an electrode 54 corresponds to the electrode 172 of the storage capacitor 170, and an anode 61 corresponds to the anode 161 of the organic EL element 160. The gate signal lines 51 are arranged in rows and the data signal lines 52 and the driving supplies 53 are arranged in columns. The storage capacitor and the emissive element layer are arranged within the area thus partitioned. The storage capacitor is formed from a semiconductor film 13 and the electrode 54. The semiconductor film 13 is connected to the data signal line 52 via a contact C1, and a gate electrode 11 is arranged between a drain 13d and a source 13s. 
A semiconductor film 43 is connected to the driving power supply 53 via a contact C2, and a gate electrode 41, which is connected to the semiconductor film 13, is arranged between a drain 43d and a source 43s. The semiconductor film 43 is connected to the anode 61 of the organic EL element via a contact C3.
FIG. 3A is a cross-sectional view along line A—A of FIG. 2. On a transparent substrate 10 is formed the semiconductor film 13, on which is covered with and formed a gate insulating film 12. On the gate insulating film 12 are formed gate electrodes 11, which branch from the gate signal line 51, and the storage capacitor electrode 54, on which is covered with and formed an interlayer insulating film 15. On the interlayer insulating film 15 is arranged the data signal line 52, which connects to the semiconductor film 13 via the contact C1. On these is covered with and formed a planarization insulating film 17.
FIG. 3B is a cross-sectional view along line B—B of FIG. 1. On the substrate 10 are laminated in sequence the semiconductor film 43, the gate insulating film 12, the gate electrode 41, and the interlayer insulating film 15, and on the interlayer insulating film 15 are formed the data signal line 52 and the driving power supply 53, on which is covered with and formed the planarization insulating film 17. On the planarization insulating film 17 is arranged an anode 61, which is connected to the semiconductor film 43 via the contact C3. On the anode 61 is arranged an emissive element layer 66, which has a laminated structure of a first hole transport layer 62, a second hole transport layer 63, an emissive layer 64, and an electron transport layer 65. A cathode 67 is arranged so as to cover them.
The anode 61 of the pattern shown in FIG. 2 is generally formed using a method in which an ITO film is first formed on the entire surface, and after forming a positive photoresist in a predetermined shape, wet etching is performed using chemicals.
However, when forming the organic EL element in this manner, the emissive element layer 66 that is formed on the anode 61 is extremely thin at approximately 200 nm so that coverage at the step portion with the planarization insulating film 17 at the edge of the anode 61 deteriorates. Thus, at the points indicated by the arrows in FIG. 4, since the vertex of the anode 61 and the vertex of the cathode 67 face each other in closer proximity than at any other location, field concentration occurs here causing a problem where the emissive layer 64 positioned between layers deteriorates rapidly. As the coverage deteriorates further, the emissive element layer 66 ruptures as shown in the figure, and the cathode 67 provided on the upper layer shorts with the anode 61 to possibly cause this pixel to be defective and not display.